(1) Field of the Invention
The present invention relates to a cold-cathode fluorescent lamp and a backlight unit that is used for a LCD (Liquid Crystal Display) apparatus.
(2) Description of the Related Art
A backlight unit, which is attached to the back of a LCD panel, is used as a light source of a LCD apparatus.
Backlight units are classified into two types: an edge-light type; and a direct-below type (see, for example, Japanese Laid-Open Patent Application No. 2000-310778).
Of these, the direct-below-type backlight units have an outer container composed of a bottom plate and side plates surrounding the bottom plate, and an opening of the outer container is covered with a diffusion plate, a diffusion sheet or the like. A plurality of fluorescent lamps are arranged in the outer container, close to the bottom. The direct-below-type backlight units are often adopted in large-scale LCD apparatuses such as 32-inch LCD televisions since it is relatively easy to achieve high brightness at the surface (hereinafter referred to as a light-emitting surface).
To achieve the high-quality image of the LCD apparatuses, such back light units are required to have high and even brightness on the light-emitting surface. Also, to enable the LCD apparatuses to occupy as little space as possible, backlight units are required to be thin.
For this reason, the inner surfaces of the outer container are coated with a material having high optical reflectance so that light radiated from the fluorescent lamps backward is reflected forward (toward the light-emitting surface) to increase the efficiency in effectively using the light fluxes radiated from the lamps (the efficiency represented by a ratio of light fluxes radiated from the light-emitting surface to light fluxes radiated from the lamp) . Also, the above-mentioned diffusion plate and the diffusion sheet are used to cause the direct light from the fluorescent lamps and reflected light to diffuse in the forward direction so that even brightness is secured all over the light-emitting surface.
Cold-cathode fluorescent lamps are often used as the fluorescent lamps. This is because the cold-cathode fluorescent lamps can be made thin with small diameters since they do not have filament coils, and this satisfies the demands for thin backlight units.
To achieve the high-quality image of the LCD apparatuses, such backlight units are further required to have high brightness.
One might think as a method for achieving the high brightness in the backlight units that the cold-cathode fluorescent lamps (hereinafter referred to as lamps) may be operated at an increased lamp current. However, this method has a problem that although the lamp light fluxes increase to some extent, the coldest-point temperature increases and exceeds an optimum range as the lamp current increases, thereby reducing the lamp light-emission efficiency (hereinafter referred to as lamp efficiency).
If, together with the increase of the lamp current, thicker lamps are used to improve the heat radiation, the coldest-point temperature will be prevented from excessively increasing. However, this method has a problem that if especially the inner diameter is increased, the distance between the tube inner wall and the center of the positive column plasma space increases, which reduces the light-emitting efficiency and prevents as much increase in the light fluxes as expected in correspondence with the increase in the lamp current from being obtained. In addition to this, the backlight units have limited space for housing the lamps. If lamps thicker than conventional ones are used in such backlight units, what is called “uneven brightness” occurs (hereinafter, such uneven brightness is referred to as “wavy uneven brightness”) in which regions on the light-emitting surface closer to the lamps have higher brightness, and regions farther away from the lamps have lower brightness. This problem can be solved by making the housing space thick, and arranging the lamps with a certain distance from the light-emitting surface. However, this method is not practical since, as described earlier, the demand for thin backlight units is strong.